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/*

* In POSIX a signal is sent either to a specific thread (Linux task)

* or to the process as a whole (Linux thread group). How the signal

* is sent determines whether it's to one thread or the whole group,

* which determines which signal mask(s) are involved in blocking it

* from being delivered until later. When the signal is delivered,

* either it's caught or ignored by a user handler or it has a default

* effect that applies to the whole thread group (POSIX process).

*

* The possible effects an unblocked signal set to SIG_DFL can have are:

* ignore - Nothing Happens

* terminate - kill the process, i.e. all threads in the group,

* similar to exit_group. The group leader (only) reports

* WIFSIGNALED status to its parent.

* coredump - write a core dump file describing all threads using

* the same mm and then kill all those threads

* stop - stop all the threads in the group, i.e. TASK_STOPPED state

*

* SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.

* Other signals when not blocked and set to SIG_DFL behaves as follows.

* The job control signals also have other special effects.

*

* +--------------------+------------------+

* | POSIX signal | default action |

* +--------------------+------------------+

* | SIGHUP | terminate |

* | SIGINT | terminate |

* | SIGQUIT | coredump |

* | SIGILL | coredump |

* | SIGTRAP | coredump |

* | SIGABRT/SIGIOT | coredump |

* | SIGBUS | coredump |

* | SIGFPE | coredump |

* | SIGKILL | terminate(+) |

* | SIGUSR1 | terminate |

* | SIGSEGV | coredump |

* | SIGUSR2 | terminate |

* | SIGPIPE | terminate |

* | SIGALRM | terminate |

* | SIGTERM | terminate |

* | SIGCHLD | ignore |

* | SIGCONT | ignore(*) |

* | SIGSTOP | stop(*)(+) |

* | SIGTSTP | stop(*) |

* | SIGTTIN | stop(*) |

* | SIGTTOU | stop(*) |

* | SIGURG | ignore |

* | SIGXCPU | coredump |

* | SIGXFSZ | coredump |

* | SIGVTALRM | terminate |

* | SIGPROF | terminate |

* | SIGPOLL/SIGIO | terminate |

* | SIGSYS/SIGUNUSED | coredump |

* | SIGSTKFLT | terminate |

* | SIGWINCH | ignore |

* | SIGPWR | terminate |

* | SIGRTMIN-SIGRTMAX | terminate |

* +--------------------+------------------+

* | non-POSIX signal | default action |

* +--------------------+------------------+

* | SIGEMT | coredump |

* +--------------------+------------------+

*

* (+) For SIGKILL and SIGSTOP the action is "always", not just "default".

* (*) Special job control effects:

* When SIGCONT is sent, it resumes the process (all threads in the group)

* from TASK_STOPPED state and also clears any pending/queued stop signals

* (any of those marked with "stop(*)"). This happens regardless of blocking,

* catching, or ignoring SIGCONT. When any stop signal is sent, it clears

* any pending/queued SIGCONT signals; this happens regardless of blocking,

* catching, or ignored the stop signal, though (except for SIGSTOP) the

* default action of stopping the process may happen later or never.

*/